The invention relates to power tools, particularly to pneumatically driven power screwdrivers of the type provided with means for monitoring the torque transmitted from a motor built into the power tool to a tool holder at the output of the tool. Arranged intermediate the drive motor and the tool holder is a clutch arrangement. The clutch arrangement is longitudinally shiftable together with the tool holder. The driven part of the clutch has a plurality of clutch depressions bounded by sloping clutch ridges. The clutch includes clutch elements, such as clutch balls, located in the clutch depressions. An axially shiftable pressure body surrounds the driving part of the clutch. Adjustable-stress spring means causes the pressure body to urge the clutch elements to the bottoms of the clutch depressions. The driving and driven parts of the clutch are immovable relative to each other in axial direction; however they are rotatable relative to each other when the torque transmitted by the clutch reaches a predetermined value causing the clutch elements to climb up the associated sloping clutch ridges out of the clutch depressions. Guided in the interior of the driving part of the clutch for axial movement relative to the driving part is a locking bolt coupled to transmit force to a release valve device when the locking bolt is axially moved. The release valve device blocks and unblocks an energy supply conduit leading to the drive motor.
In a known power tool of this construction the clutch elements are clutch balls and the pressure body is a pressure plate. Also, the clutch arrangement simultaneously serves to hold the tool, e.g., a screwdriver element or the like. The driving part of the clutch has a plurality of radial bores. Locking balls are arranged inside these radial bores and bear upon the surface of the locking bolt. The locking bolt is provided with a conical surface which pushes the locking balls radially outward into depressions on the pressure plate. The positions of the locking balls are fixed when they are received within these depressions. The driving part is tube-shaped and connected with the motor via an intermediate shaft. The driving part is provided with an integral flange-shaped disk provided with a plurality of through-passages in which the clutch balls are positioned. The first spring means, which holds the clutch balls in place until the threshold torque is reached, can be a dish spring arrangement.
With this known power tool, when the predetermined threshold torque is reached, the clutch balls rise up the sloping ridges of the clutch depressions on the driven part of the clutch and move completely out of these clutch depressions, so that the driven and driving parts of the clutch become momentarily disengaged. As the clutch balls climb out of their clutch depressions, they push the pressure plate back against the force of the first spring means (the threshold-torque-determining spring), and then slip over into the respective neighboring clutch depressions. However, as soon as the backward shifted pressure plate unblocks the radial bores in the driven part of the clutch, a second spring (the locking spring) causes the locking bolt to press, specifically with its conical surface, against the locking balls so as to force them radially outward into depressions on the pressure plate. Simultaneously, the forwardly shifted locking bolt through the intermediary of a linkage rod closes the rease valve device, thereby causing the drive motor to stop.
With that arrangement, the locking balls must hold the pressure plate against the force of the torque-threshold-determining spring, which presses in opposition through the intermediary of the pressure plate. In certain circumstances this can shorten the useful life of the locking balls and cause excessive wear of the locking edges of the tube-shaped driving part against which the locking balls press.